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  ZooKeeper 监视点详解
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  <p>应用程序需要知道 ZooKeeper 节点状态的情况很常见，如主从模式中从节点需要知道主节点是否崩溃。</p>
<p>轮训的方式很低效，尤其是在期望变化频率很低时。</p>
<p>ZooKeeper 提供了监视点（watch）这一处理节点变化的重要机制，客户端对指定 znode 节点注册通知请求，在节点状态发生变化时就会收到一个单次的通知。</p>
<a id="more"></a>
<h2 id="单次通知"><a href="#单次通知" class="headerlink" title="单次通知"></a>单次通知</h2><p>监视点是一个单次的触发器，应用对某个节点注册了一个监视点，会在该节点第一次触发事件时向应用发送通知。</p>
<p>监视点与会话关联，当客户端与一台 ZooKeeper 服务器断开连接后，会连接到集合中另一台服务器，同时将未触发的监视列表发送到服务器。如果所监视的节点已经发生变化，会向客户端发送通知，否则会在新的服务器上注册监视点。</p>
<p>单次触发的通知可能会丢失事件，比如在收到通知、注册新的监视点之间的这段时间所发生的事件。不过丢失事件通常不会造成影响，节点变化可以通过注册新监视点时读取节点状态来捕获。</p>
<h2 id="设置监视点"><a href="#设置监视点" class="headerlink" title="设置监视点"></a>设置监视点</h2><p>ZooKeeper 中的所有读操作均可设置监视点，包括 getData、getChildren 和 exists。</p>
<p>使用监视点需要实现 Watcher 接口，重写 void process(WatchedEvent event) 方法。其中 WatchedEvent 包含 ZooKeeper 的会话状态（KeeperState）和事件类型（EventType）。</p>
<p>事件类型包括 NodeCreated、NodeDeleted、NodeDataChanged、NodeChildrenChanged 和 None。前 3 个类型涉及单个 znode 节点，第 4 个即 NodeChildrenChanged 涉及所监视节点的子节点。None 表示 ZooKeeper 会话状态发生变化。</p>
<table>
<thead>
<tr>
<th>事件类型</th>
<th>设置监视点方式</th>
</tr>
</thead>
<tbody>
<tr>
<td>NodeCreated</td>
<td>通过 exists 调用设置监视点</td>
</tr>
<tr>
<td>NodeDeleted</td>
<td>通过 exists 或 getData 调用设置监视点</td>
</tr>
<tr>
<td>NodeDataChanged</td>
<td>通过 exists 或 getData 调用设置监视点</td>
</tr>
<tr>
<td>NodeChildrenChanged</td>
<td>通过 getChildren 调用设置监视点</td>
</tr>
</tbody>
</table>
<h2 id="原子操作"><a href="#原子操作" class="headerlink" title="原子操作"></a>原子操作</h2><p>ZooKeeper 在 3.4.0 版本添加了原子操作的特性，通过 multiop 可以原子性地执行多个 ZooKeeper 操作，这些操作要么全部成功，要么全部失败。</p>
<p>使用 multiop 时，首先创建出包含 ZooKeeper 操作的 Op 对象，再把多个 Op 对象放到 Iterable 类型的对象中供客户端调用，如：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function">Op <span class="title">deleteZnode</span><span class="params">(String path)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> Op.delete(path, -<span class="number">1</span>);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function">Op <span class="title">createZnode</span><span class="params">(String path, String data)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> Op.create(path, data.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">List&lt;OpResult&gt; results = zk.multi(Arrays.asList(deleteZnode(<span class="string">"/a"</span>), createZnode(<span class="string">"b"</span>, <span class="string">"data"</span>)));</span><br></pre></td></tr></table></figure>
<p>multi 方法同样有异步版本，同步和异步方法的定义如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> List&lt;OpResult&gt; <span class="title">multi</span><span class="params">(Iterable&lt;Op&gt; ops)</span> throw InterruptedException,KeeperException</span>;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">multi</span><span class="params">(Iterable&lt;Op&gt; ops, MultiCallback callback, Object context)</span></span>;</span><br></pre></td></tr></table></figure>
<p>Transaction 封装 multi 方法，提供了更简单的调用方式，可以通过创建 Transaction 对象来添加操作、提交事务。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">List&lt;OpResult&gt; results = zk.transaction().delete(<span class="string">"/a"</span>, -<span class="number">1</span>).create(<span class="string">"/b"</span>, <span class="string">"data"</span>.getBytes()</span><br><span class="line">        , ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT).commit();</span><br></pre></td></tr></table></figure>
<p>multi 提供的另一个功能是检查 znode 的版本号，在输入的 znode 节点与版本号不匹配时，multi 会调用失败。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Op <span class="title">check</span><span class="params">(String path, <span class="keyword">int</span> version)</span></span>;</span><br></pre></td></tr></table></figure>
<h2 id="消除隐藏通道"><a href="#消除隐藏通道" class="headerlink" title="消除隐藏通道"></a>消除隐藏通道</h2><p>ZooKeeper 的状态会在所有服务器间互相复制，服务端对状态变化的顺序会达成一致，并使用相同的顺序对状态进行更新。但事实上服务端很少同时执行更新操作，如果通过隐藏通道进行通信可能会出现状态不一致的现象。</p>
<p>比如 Client-1 连接到 Server-1，Client-2 连接到 Server-2，/z 节点中的数据由 a 变为 b。</p>
<p>Server-1 更新状态会向 Client-1 发送通知，Client-1 收到通知后向 Client-2 发送 /z 节点变化的消息，Client-2 再通过 Server-2 进行操作，如果此时 Server-2 还没有对 /z 节点的状态进行更新，就会读取到过期的数据。</p>
<p>Client-1 向 Clinet-2 发送消息，就是隐藏的通道，会导致错误发生，正确的做法是 Client-2 只通过 ZooKeeper 服务端接收消息，消除隐藏通道。</p>
<h2 id="避免在同一个节点设置过多监视点"><a href="#避免在同一个节点设置过多监视点" class="headerlink" title="避免在同一个节点设置过多监视点"></a>避免在同一个节点设置过多监视点</h2><p>当节点发生变化时，会向设置监视点的客户端发送消息，如果对同一节点设置过多的监控点，就会在节点状态变更时出现发送消息的高峰，可能会对性能造成影响。</p>
<p>条件允许的话，最好对节点设置少量的监视点，理想情况下只设置一个。</p>
<p>比如多个客户端争相获取一个锁：可以一个客户端通过创建临时节点获取锁，其他客户端对这个节点设置监视点；也可以改为让客户端创建有序临时节点，最小序号的客户端获取锁，其他客户端只对比自己序号小 1 的节点设置监视点。</p>
<p>一个监视点会占用服务端约 300 字节的内存，开发时需要注意监视点数量。</p>
 
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